Breathing insufficiencies and irregularities may occur in conjunction with or as a result of a variety health related disorders and may further cause or exacerbate health disorders. Such breathing insufficiencies and irregularities may include, for example, hyperventilation, hypoventilation, apnea, and other related breathing disorders. Hyperventilation, which results in hyperoxia, is a condition in which the respiratory rate is pathologically high or is above a desired rate. Hyperventilation may occur due to pulmonary edema or excess fluid built up in the lungs and may ultimately result in apnea episodes. Hypoventilation is a condition in which the respiratory rate is pathologically low or below a desired rate. Apnea (absence of breathing) is a breathing disorder most typically occurring during sleep that can result from a variety of conditions. Sleep apnea typically results in some sort of arousal or wakefulness following cessation of breathing.
Sleep disordered breathing disorders include two types of sleep apnea: obstructive sleep apnea (partial apnea or obstructive apnea) and central sleep apnea. Obstructive sleep apneas result from narrowing of the pharynx with out-of-phase breathing in an effort to create airflow, whereas central sleep apnea arises from reductions in central respiratory drive. During obstructive sleep apnea, respiratory effort increases. In central sleep apnea, respiratory movements are absent or attenuated but in phase.
Disordered breathing may contribute to a number of adverse cardiovascular outcomes such as hypertension, stroke, congestive heart failure, and myocardial infarction. Sleep-related breathing disorders, especially central sleep apnea, have been found to have a relatively high prevalence in patients with heart failure and may have a causative or influencing effect on heart failure. In about 50% of patients with stable congestive heart failure, there is an associated sleep disordered breathing, predominantly central sleep apnea with a minority having obstructive sleep apnea. Furthermore, sleep related breathing disorders are believed to be physiologically linked with heart failure. Central sleep apnea is a known risk factor for diminished life expectancy in heart failure. It is also believed that in view of this link, treatment aimed at relieving sleep related breathing disorders may improve cardiovascular outcomes in patients with heart failure.
Pulmonary edema, a condition in which there is excess fluid in the lungs and often found in heart failure patients, is believed in some circumstances to lead to hyperventilation and hyperoxia or apnea. Most heart failure patients with central sleep apnea, when lying flat, tend to have central fluid accumulation and pulmonary congestion, which stimulates vagal irritant receptors in the lungs to cause reflex hyperventilation. Central Sleep Apneas usually are initiated by reduction in PCO2 resulting from the increase in ventilation. When PCO2 falls below the threshold level required to stimulate breathing, the central drive to respiratory muscles and airflow cease or diminish significantly and apnea (or attenuated breathing) ensues until the PCO2 rises again above the threshold required to stimulate ventilation. Often spontaneous arousal occurs with apnea.
Currently a number of methods are used to treat sleep apnea. For example, supplemental oxygen such as, e.g., with a nasal ventilator, has been used to relieve symptoms of sleep apnea. Non-invasive airway pressure including continuous positive airway pressure (CPAP), bivalve and adaptive pressure support servo-ventilation have been used to treat central sleep apnea and obstructive sleep apnea with varying results. Another method to treat central sleep apnea is using aggressive diuresis to lower cardiac filling and beta-blocker and angiotensin-converting enzymes. However, this treatment does not lead to an optimum therapy since excessive use of diuretics leads to renal complications and patient discomfort.
A method and apparatus for treatment of obstructive sleep apnea has been proposed where an implantable pulse generator stimulates a nerve in the upper airway tract of a patient to elicit a contraction by an innervated muscle through the provision of electrical stimuli. The stimulator is intended to treat the obstructed airway passage to permit breathing. The pulse generator is attached to electrodes placed on the patient's diaphragm for sensing the respiratory effort of a patient whereupon the stimulation is adjusted. The method and apparatus do not provide a satisfactory treatment of central sleep apnea.
Phrenic nerve stimulation has been used to stimulate the diaphragm throughout an overnight period to treat sleep apnea. The device used was turned on at night to stimulate the nerve continuously and then turned off during the day. However, this device was not adapted for situations where patients would breath spontaneously.
Accordingly it would be desirable to provide a method and apparatus for treating breathing disorders such as apnea, and hypoventilation, and especially central sleep apnea. Furthermore it would be desirable to provide treatments for breathing related disorders related pulmonary edema and conditions in heart failure patients.